The present invention relates to low color block copolymers modified with acid compounds. More specifically, the invention relates to a process for improving the color of block copolymers of conjugated dienes and vinyl aromatic hydrocarbons to which at least one acid compound had been grafted.
U.S. Pat. No. 4,578,429 discloses selectively hydrogenated block copolymers of conjugated dienes and vinyl aromatic hydrocarbons which have been modified with acid compounds or derivatives thereof. Such polymers have been found to be useful to increase adhesion to fibers and fillers, substrates in adhesives, sealants and coatings, decorations such as printing inks, paints, primers and metals, etc. First, the block copolymer is made according to a process such as described in U.S. Pat. No. Re. 27,145. After hydrogenation, the polymer is subjected to an acid wash with sulfuric acid or phosphoric acid to oxidize the hydrogenation catalyst and facilitate extraction. Then, an acid group or a derivative, such as maleic anhydride, is grafted onto the polymer by a reaction of the acid group with the polymer in the melt.
While these polymers have been found to be quite useful as described above, their usefulness has been limited because they tend to develop a yellow color. The color makes them unacceptable to many customers for use in films, color-matched compounds, and clear applications. Thus, it would be advantageous to be able to produce such functionalized block copolymers which permanently exhibit a low color.